2606 WHEELER: THE PROPAGATION OF FLAME INCCXLII1.-The Propagation of Flame in Mixtui-es ofBy RICHARD VERNON WHEELER.IN the course of their well-known remarches on the combustion ofexplosive gaseous mixtures, Mallard and Le Chatelier (Ann. desMines, 1883, [viii], 4, 274) studied the propagation of flame inmixtures of methane and air contained in horizontal tubes. So faras such mixtures are concerned, the general cbnclusions drawn bythem regarding the manner in which flame is propagated were asfollows.When the mixture contained in a horizontal tube closed a t oneend and open a t the other is ignited a t the open end, the flametravels for a short distance a t a uniform speed. This “uniformmovement ” is followed by a “vibratory movement,” in the courseof which the flame travels backwards and forwards in an irregularmanner, the mean speed from point t o point along the tube beingusually greater than that of the “uniform movement.’’ Thesevibrations usually continue t o the end of the tube, but sometimesduring a particularly violent, vibration the flame may be extin-guished, owing t o the rnixing of burnt gases with the unburntmixture.When the mixture is ignited a t the closed end of the tube theflame travels, in shori; tubes at all events, with increasing speedtowards the open end.I n the course of investigations on mine explosions, carried out,in the first instance, for the Mining Association of Great Britain,and, latterly, a t the Home Office Experimental Station, the neces-sity arose for repeating Mallard and Le Chatelier’s experimentsregarding mixtures of methane and air.The present paper deals with the ‘‘ uniform movement,” the speedof which is the normal speed of propagation of flame by conductionof heat from layer to layer of the mixture, and is constant for agiven mixture a t a given temperature and pressure.Mallard and Le Chatelier made a complete.study of how far thediameter, length, and material of the tubes influenced the speedand duration of the uniform movement in many gaseous mixtures,with the object of determining the limiting dimensions requisitet o ensure that the true speed-the speed that would be obtainedin a mixture of indefinite extent-should be determined.Giventhe right dimensions of tubes, the material of which they weremade did not appreciably affect the speeds.Repetition of MallardMethane and Ah*. The ‘‘ i7r~ifoor.m Movement.MlXTURES OF METHANE AND AIR. 2607and Le Chatelier’s experiments regarding these experimental condi-tions has confirmed their results.The diameter of tube necessary t o avoid cooling by the walls,and consequent retardation of the flame, was found to be greaterthe slower the speed of the flame. For the most slowly movingflames in mixtures of methane and air it tube of a t least 5 cm.diameter is necessary. The speed of travel of flame in a tube 9 cm.in diameter is slightly greater than in a tube 5 cm. in diameter.The duration of the uniform movement, which varies with eachmixture, increases with the diameter and length of the tube up t oa certain maximum, after which increase in length makes no appre-ciable difference.I n a tube 5 cm. in diameter and 6 metres longthe uniform movement in all mixtures of methane and air extendsover a dist.ance of about 150 cm., whereas in a tube of the samediameter and 2 metres long the distance travelled by the flame a ta uniform speed may be less than 50 cm.For their experiments Mallard and Le Chatelier used tubes5 cm. in diameter and 1 metre long, and measured the speed oftravel of flame over tho first 50 cm. The length of the tube wasinsufficient to ensure that the measurements of the speed of theflame would not include part of the “vibratory movement,” a factwhich they themselves realised (Zoc. cit., p.317). Their measure-ments for the same mixture show, in consequence, rather widevariations. Their experiments were further vitiated by the factthat the methane used was prepared from sodium acetate (“i1exhalait une forte odeur d’acetone ”). Such methane ” maycontain as much as 10 per cent. of nitrogen, 10 per cent. ofunsaturated hydrocarbons, and 2 or 3 per cent. of hydrogen.*The conclusions drawn by Mallard and Le Chatelier were:(1) The speed of the uniform movement increases regularly withthe percentage of methane up to a certain maximum, after whichi t decreases regularly. The curve obtained on plotting speeds asordinates and percentages of methane as abscissze is thus repre-sented by two straight lines meeting a t a point. Their curve(from Plate VIII of their paper) is reproduced in Fig.1.(2) The maximum speed is obtained, not with that mixturecontaining the quantity of methane required for complete com-bustion, namely, 9.4 per cent., but with a mixture containingabout 12 per cent. of methane. Le Chatelier (“Leqons sur lecarbone,” Paris, 1908) explains this result by assuming that thespeed of propagation of flame during the uniform movementdepends, not only on the temperature of combustion of the mixture,but on its thermal conductivity, which is greater the greater the* Compare Hauser, “ Leqons sur le grison,” Madrid, 19082608 WHEELER: THE PROPAGATION OF FLAME INproportion of methane present. The thermal conductivities of airand of methane are 5.22 x 10-5 and 6-47 x 10-5 respectively.Fresh determinations, made in the manner described in theexperimental portion of this paper, do not bear out Mallard andLe Chatelier’s results.The form of curve obtained on plottingspeeds as ordinates and percentages of methane as abscisse isshown in Fig. 1.It will be seen that there is practically no difference between thespeeds attaineld in mixtures containing from 9-45 t o 10.55 percent. of methane,* such differences as there are being probablywithin the limits of experimental error.Near the lower- and higher-limits of inflammability, which, forhorizontal propagation, are 5.4 and 14.3 per cent. respectively, theFIG. 1.6 5’4 6 7 8 9 10 11 12 13 1414‘” 15 16 17AIcthnne in fit-ednnzp-air mixture, per cent.curve flattens, more noticeably towards the higher limit, andbecomes, ultimately, nearly horizontal.It will be understood,therefore, that a prolongation of either “limb” of the curve 60as t o cut the zero velocity ordinate, as done by Nallard andLe Chatelier to determine the theoretical limits of inflammability,is not justifiable.Vibrations were not developed by the] flames in all the mixtures.I n those containing more than 12.5 o r less than 5.8 per cent. ofmethane the flame usually travelled a t a uniform or slightly decreas-* This conclusion is confirmed by another series of experiments in whichdifferent mixtures of methane aud air were ignited a t the centre of a large sphericalexplosion vessel. The tirrie that elapsed between the moment of ignition and thefirst indication of pressure on the sides of the vessel was less the higher the per-centage of methane in the mixture up to 9.5 per cent.methane, after which itremained practically constant up to 11 per centMIXTURES OF METHANE AND AIR. 2609iiig speed throughout the length of the tube, although sometimesslight vibrations were noticeable in all but the " limit-mixtures."I n these latter the speed of travel of flame was quite uniformthroughout,, and was the same for both the higher- and lower-limitmixtures.As noted in a previous paper (this vol., p. 2593), the flameF I G . 2 .travelling horizontally in a 5.4 per cent. methane-air mixture,contaiued in a tube 5 em. in diameter, occupies only the upperpart of the tube. The flames in the other mixtures of methane andair, including the higher-limit mixture, completely filled the cross-section of the tube, the front of the flame (during the uniformmovement) being shaped as shown in Fig. 2.The faster the speedof the flame the blunter was its front.EXPERIMENTAL.The arrangement of glass tubes is shown in Fig. 3. Three lengthsof t'ube of 5 cm. internal diameter, each 2 metres long, were joinedtogether by broad pieces of stout rubber tubing, and supportedhorizontally in a straight line. Each end of the complete lengt2610 WHEELER: THE PROPAGATION OF FLAME I Nof 6 metres was flanged and ground t o receive flanged end-pieces,which were held in position by metal clips. Each end-piece wasfitted with a wide-bore three-way tap.Glass-covered platinum elec-trodes reaching to the centre of the tube, leaving a spark-gap of3 mm., were fused 4 cm. from one end.Another tube, similarly arranged, but of 9 cm. internaldiameter, was used for a separate series of experiments.“ Screen-wires ” of copper 0.025 mm. in diameter were threadedvertically across the tube through fine holes pierced through thewalls a t certain points, the holes being afterwards covered byadhesive plaster. In order t o avoid including in the measurementsof the speed of the flame any impetus that might be given by theigniting spark, the first screen-wire was fixed 40 em. from the pointof ignitlon. OGher screen-wires were fixed 50, 100, 200, 300, and400 cm. respectively from the first.The method of recording the time of passage of flame along thetube was erectrical.Each screen-wire carried a small electriccurrent, the interruption of this current when the flame meltedthe wires being recorded by the movement of an electro-magnet.It was important to avoid error due to latency or “timelag” ofindividual electromagnets. An instrument, which can be termedan automatic commutator, was therefore designed to enable eachsuccessive break in circuit to be recorded by the same electromagnet.This instrument is operated in the following manner:One terminal of the battery supplying the electric current isconnected to the brush, -4, of the commutator (Fig. 3), and a leadfrom the other terminal of the battery conducts the current t o theelectromagnet of the chronograph, so that its armature is attracted.The current tEen passes by a lead to the electromagnet on thecommutator, and that armatare is also attracted; the lead carryingthe current then goes t o one terminal of the scre’en-wirea on theexplosion-tube one after the other; the other terminal of eachscreen-wire is connected to the corresponding stud on the commu-tator by separate leads.Supposing the brush, A , to ue resting on No.1 stud (the positionthat it occupies a t the beginning of an experiment), the current isthen flowing through the chronograph electromagnet, the commu-tator electromagnet, and No. 1 screen-wire; then through thebrush, A , back t o the battery. Suppose now that the flame passesalong the tube’ and melts screen-wire No.1; the chronographelectromagnet rolease; its armature, and the pen it carries makesa mark on the moving surface; a t the same time the armature, B,of the commutator electromagnet is released, and the anchor-escapement, C, attached to the armature, is moved. This allows thMIXTURES OF METHANE AND AIR. 2611coiled spring, D, to pull the scapewheel, 3, round by the cord, F ,which is wound on a drum attached to the axis of the scape-wheel.The brush, A, then moves on 50 stud No. 2, and the current atonce begins to flow through screen-wire No. 2; the chronographelectromagnet and the commutst.or electromagnet, and the arma-tures of both these are again attracted; the pen on the chrono-graph is moved back to its former position, as also are the armature,13, and the escapement, C, whilst the brush, A , moves a littlefurther on to stud No.2. When the flame reaches No. 2 screen-wire the same cycle is repeated and so on for as many screens asmay be required, all the interruptions of circuit being recorded bythe one pen on the chronograph.*The chronograph used was the laboratory chronograph of theCambridge Scientific Instrument Company, the speed of travel ofthe moving surface (a spool of Morse paper) being recorded by a$-second contact-clock.Xethod of Ccnducting a2z Experiment.-The mixtures ofmethane and air were made in a 140-litre gas-holder over waterrendered slightly alkaline by potassium hydroxide. A rapidcurrent of the mixture was passed through the explosion-tube untilthe gases entering and leaving had the same composition, a8 shownby explosion-analyses of samples taken through the three-way taps.A volume of mixture equal to about six times the volume of thetube was found to be ample for sweeping out all the air containedin the tube.All electrical connexions through the screen-wires and chrono-graph having been established, the left-hand end-piece of theexplosion-tube was removed (by sliding it downwards) and themixture ignited a t the now open end by passing an induction-coilspark.The methane used was a particularly pure supply of fire-dampfrom a " blower" a t a colliery in South Wales, whence it wasobtained compressed in cylinders.Analysis, after removal of0.8 per cent. of carbon dioxide, showed it t o contain 97.4 per cent.of methane, 2.3 per cent.of nitrogen, and 0.3 per cent. of otherimpurities (carbon monoxide and ethylene). It contained nohydrogen or ethane.* A somewhat detailed description of this device has been given in the belief thatit may prove of value to other workers. The author has fvund it adequate formeasuring the speed of the rapidly inoviiig fl'rmes of coal dust explosioiis and coal-gas and air explosions in large galleries. Its effectiveness depends essentially onthe rapidity with which the brush of the commutator can be made to move fromone stud t o the next ; by snitable proportioiling and adjustment of the movingparts and regulation of the electric current passing through the magnets, the timetaken for the brush to move from stud to stud can be niade as little as &th second261 2 WHEELER : THE PROPAGATION OF FLAME. ETC ..Results of E'xperzments.-The rssulb of all the determinationsmade of the speed of the uniform movement in different mixturesare given in the table that follows .As a general rule, the uniformmovement extended for a distance of 150 cm . from the point ofignition. so that from each experiment wit'h a particular mixturetwo determinations of the speed were obtained (between No . 1 andNo . 2. and between No . 2 and No . 3 screen-wire respectively) .Some of the more rapidly-moving flames. in mixtures containingbetween 9.5 and 11.0 per cent . of methane. began to vibrate justbefore reaching tile third screen-wire; in such cases only the speedbetween screen-wires Nos . 1 and 2 was taken as being that of theuniform movement .Methane in fire-damp-airmixture. per cent .Speed of " uniform movement " offlame. ern . per second .5-40 ........................ 36.5, 36.0, 35.5, 35.5, 35.5, 35.5, 36.0,5.60 ........................ 37.0, 37.0.5-85 ........................ 40. 40.5, 40.5, 40.5.6.25 ........................ 46. 46. 45.5, 45. 45.5, 45. 45.5.6.80 ........................ 56. 55. 55. 55 .36.0, 35.5 .6.75 ........................ 54. 54 .7.10 ........................ 61. 59. 61. 59 .7-70 ........................ 77. 77. 75. 75 .9.10 ........................ 105. 104. 106. 104 .9.20 ........................ 108. 109 .9.45 ........................ 110.110. 110. 110 .9.60 ........................ 111. 111 .9.80 ........................ 111. 112 .8-36 ........................ 91. 90 .8.80 ........................ 100. 100. 99. 100 .10.00 ........................10.60 ........................ 109. 109. 109 .10.90 ........................ 102. 102. 101 .11-00 ........................ 100. 98. 99. 99 .11.20 ........................ 93. 92 .11.50 ........................ 84. 84. 83. 83. 85. 84 .12.10 ........................ 62.5, 62 .12.50 ........................ 50. 50. 49 .12.65 ........................ 49. 47. 48. 46 .13.00 ........................ 42.5, 42. 42. 42 .13.05 ........................ 41. 40.5.13.30 ........................ 39. 38. 39. 38. 38. 38. 38 .13-80 ........................37. 36.5.14.30 ........................ 36.0, 35.5, 35.5.112. 110. 112. 113. 112. 110. 109. 113 .For Lne determinations of the speed of travel of flame in thehigher-limit mixture pure methane was used. since the 2.3 per cent .of nitrogen contained in the fire-damp slightly affected the higherlimit. whereas it had no appreciable effect on the speed of travelof flame in the other mixtures (compare this vol., p . 2596) .A similar series of clet'erminations was made. using an explosion-tube of 9 cm . internal diameter . The speeds were from 5 t o 10 cm .per second greater than those of corresponding mixtures in thetube 5 cm . in diameter . The shapes of the curves connecting speedVOLATILE OIL FROM THE LEAVES OF UAROS&IA VENUSTA. 2613with percentages of methane, and the limits of inflammability, weretlie same in both series of experiments.The' propagation of flame in mixtures of methane and air, andin mixtures t o which nitrogen has been added, has been furtherstudied. Ail account of the work will be communicated later t othe Society.Adde?idum.Since this paper was prepared an account has appeared of experi-ments on the samel subject by A. Parker and A. V. Rhead (thisvol., p. 2150). It is surprising to find that these aut'hors are appa-rently unacquainted with Mallard and Le Chatelier's completeresearches dealing with the " uniform " and " vibratory " move-ments during the propagatior of flame in gaseous mixtures con-tained in glass tubes, as outlined in the present paper. Theirresults are interesting in that they emphasise the necessity, pointedout by Mallard m d IJe Chatelier, of employing tubes of amplediameter when conducting experiments of this nature; the tubesthey used were of too sniall a diameter t o enable them t o determineeither the true character of the speed-percentage curve or the limitsof inflammability